Prognosis of Idiopathic Downbeat Nystagmus

Downbeat nystagmus (DBN) is a common form of acquired involuntary ocular oscillation overriding fixation. Little is known about the natural evolution of DBN in idiopathic cases. Therefore, eye movements were recorded in seven patients with idiopathic DBN by search‐coil technique over a course of up to six years. Neither the slow‐phase velocity (SPV) nor the dependency of the SPV on gaze position as an indicator of dysfunction of the oculomotor velocity‐to‐position integrator changed significantly over the time course investigated in this study.

Keywords: downbeat nystagmus; scleral search coil; flocculus

Downbeat nystagmus (DBN) is a frequent form of acquired involuntary ocular oscillation overriding fixation. It is commonly caused by cerebellar disorders involving the (para‐) floccular lobe, such as cerebellar degenerative disease and posterior fossa infarction, and—rarely—brainstem lesions. In many cases, no underlying pathology can be detected, the so‐called idiopathic DBN.[1] In the case of secondary DBN, the prognosis depends on the course of the underlying disease. However, little is known about the natural evolution of DBN in idiopathic cases. Therefore we recorded eye movements in seven patients with idiopathic DBN by search‐coil technique over a course of up to six years. We show that neither the slow‐phase velocity (SPV) nor the dependency of the SPV on gaze position as an indicator of dysfunction of the oculomotor velocity‐to‐position integrator[2] changes significantly over the time course investigated in this study.

Seven patients with idiopathic DBN (three men, mean age at the time of the first recording 68 years) took part in the study. Three‐dimensional eye and head movements were recorded at 1 kHz with dual search coils (Skalar, Delft, The Netherlands; Remmel Systems, Ashland, Massachusetts, USA) placed on the left eye and the forehead at different points in time during a period of 2 to 6 years (mean 4 years). The three‐dimensional SPV of DBN and its dependency on eye position were evaluated in different eye positions, as described previously.[2] Tests were performed in darkness, with a laser target point being visible throughout the whole experiment and with a flashed target that was visible for less than 100 ms. T‐test for dependent samples was used to detect differences in SPV and its dependency on eye position between the first and last recording.

Out of seven patients, two had three recordings (baseline, two, and five years) and five had two recordings (baseline and two, three, five, or six years afterward). The individual courses of the patients' DBN SPV and its dependency on the vertical eye position are depicted in Figure 1. Mean SPV at baseline was −3.17 (±2.32) °/s. Mean SPV at the final recording was −1.75 (±3.90) °/s. There was no significant change of SPV (t‐test for dependent samples, P= 0.56, T=−0.62) or dependency of SPV on gaze direction (P= 0.57, T= 0.60).

Graph: 1 The individual courses of (top) the patients' Downbeat nystagmus (DBN) slow‐phase velocity (SPV) and (bottom) its dependency on the vertical eye position are depicted. Round dots represent individual measurements.

So far, little is known about the natural evolution of DBN. In this study we were able to show that oculomotor findings in patients with idiopathic DBN do not change significantly over a period of up to 6 years.

The exact pathophysiology of idiopathic DBN is as yet unknown. An inherent asymmetry of peripheral vestibular input has been proposed, as has central imbalance in the vertical vestibulo‐ocular system.[[3]] Other authors suggest an imbalance of the smooth‐pursuit system or a mismatch of the coordinate systems of the saccadic burst generator and the neural eye‐velocity‐to‐position integrator.[[2], [6]] However, the cerebellar floccular lobe seems to play a major role in the generation of DBN.[7]

Our results suggest different scenarios concerning the underlying pathological process. Either it progresses very slowly, requiring longer periods of follow‐up to detect a difference in the oculomotor findings. Alternatively, clinically overt DBN may be the final stage of a degenerative process after all compensatory mechanisms have failed. Follow‐up of a larger group of patients will be necessary to clarify this issue. This follow‐up may also lead to detection of subgroups of patients with distinct courses of the disease. These might reveal different etiologies of so‐called "idiopathic" DBN.

The authors declare no conflicts of interest.